Light-emitting diode with compensating conversion element and corresponding conversion element

ABSTRACT

A light-emitting diode includes a light-emitting diode chip which emits primary radiation in a spectral range of blue light during operation; a conversion element including a first phosphor and a second phosphor which absorbs part of the primary radiation and re-emits secondary radiation, wherein the first phosphor has, in an absorption wavelength range (Δλ ab ), an absorption that decreases as the wavelength increases, and the second phosphor has, in the same absorption wavelength range (Δλ ab ), an absorption that increases as the wavelength increases; the primary radiation includes wavelengths that lie in the absorption wavelength range (Δλ ab ); and the light-emitting diode emits white mixed light including primary radiation and secondary radiation and having a color temperature of at least 4000 K.

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/EP2010/059180, withan international filing date of Jun. 29, 2010 (WO 2011/012388 A1,published Feb. 3, 2011), which is based on German Patent Application No.10 2009 035 100.00, filed Jul. 29, 2009, the subject matter of which isincorporated by reference.

TECHNICAL FIELD

This disclosure relates to a light-emitting diode, particularly to aconversion element for a light-emitting diode.

BACKGROUND

WO 2008/020913 A2 describes a conversion element for generatingwarm-white mixed light.

However, it could be helpful to provide a light-emitting diode whichgenerates electromagnetic radiation whose color locus is particularlyinsensitive to fluctuations in operating current and/or operatingtemperature of the light-emitting diode. In particular, it could behelpful to provide a light-emitting diode suitable for generatingcold-white light.

SUMMARY

I provide a light-emitting diode including a light-emitting diode chipwhich emits primary radiation in a spectral range of blue light duringoperation, a conversion element including a first phosphor and a secondphosphor which absorbs part of the primary radiation and re-emitssecondary radiation, wherein the first phosphor has, in an absorptionwavelength range (Δλ_(ab)), an absorption that decreases as thewavelength increases, and the second phosphor has, in the sameabsorption wavelength range (Δλ_(ab)), an absorption that increases asthe wavelength increases, the primary radiation includes wavelengthsthat lie in the absorption wavelength range (Δλ_(ab)), and thelight-emitting diode emits white mixed light including primary radiationand secondary radiation and having a color temperature of at least 4000K.

I also provide a conversion element for a light-emitting diode, theconversion element provided to absorb a primary radiation and emit asecondary radiation, including a first phosphor and a second phosphor,wherein the first phosphor has, in an absorption wavelength range(Δλ_(ab)), an absorption that decreases as the wavelength increases, andthe second phosphor has, in the same absorption wavelength range(Δλ_(ab)), an absorption that increases as the wavelength increases, andwavelengths of the maximum emission intensity of the first and secondphosphors differ by at most 20 nm.

I further provide a light-emitting diode including a light-emittingdiode chip which emits primary radiation in a spectral range of bluelight during operation, a conversion element including a first phosphorand a second phosphor which absorbs part of the primary radiation andre-emits secondary radiation, wherein the first phosphor has, in anabsorption wavelength range (Δλ_(ab)), an absorption that decreases asthe wavelength increases, and the second phosphor has, in the sameabsorption wavelength range (Δλ_(ab)), an absorption that increases asthe wavelength increases, the primary radiation includes wavelengthsthat lie in the absorption wavelength range (Δλ_(ab)), thelight-emitting diode emits white mixed light including primary radiationand secondary radiation and having a color temperature of at least 4000K, the first phosphor is based on europium as a luminous center and thesecond phosphor is based on cerium as a luminous center, and the weightratio of the first phosphor to the second phosphor is 0.60 to 1.5.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2A, 2B, 3A, 3B and 4 to 9 are graphical illustrations oflight-emitting diodes and conversion elements.

FIGS. 10A to 10D are schematic sectional illustrations showing differentexamples of light-emitting diodes and conversion elements.

DETAILED DESCRIPTION

We provide a light-emitting diode that may comprise a light-emittingdiode chip. The light-emitting diode chip has, for example, asemiconductor body composed of an inorganic semiconductor material. Thesemiconductor body comprises one or a plurality of active zones providedfor generating electromagnetic radiation. During operation, thelight-emitting diode chip preferably emits primary radiation in thespectral range of ultraviolet radiation and/or blue light. That is tosay that, during operation of the light-emitting diode chip, ultravioletradiation and/or blue light is emitted by the light-emitting diode chip.The electromagnetic radiation emitted by the light-emitting diode chipis in this case the primary radiation of the light-emitting diode.

The light-emitting diode may comprise a conversion element. Theconversion element is provided for absorbing at least part of theprimary radiation of the light-emitting diode chip. That is to say that,during the operation of the light-emitting diode, the light-emittingdiode chip emits the primary radiation and the latter passes at leastpartly into the conversion element, by which it is in turn partlyabsorbed. The conversion element is excited by the absorbed primaryradiation to re-emit a secondary radiation. That is to say that, duringoperation of the light-emitting diode, the conversion element re-emitssecondary radiation. In this case, the secondary radiation preferablyhas wavelengths that are greater than wavelengths of the primaryradiation.

The conversion element may comprise a first phosphor and a secondphosphor. That is to say that the conversion element is not formed witha single phosphor suitable for absorbing and re-emitting electromagneticradiation, but rather with two different phosphors. In this case, theconversion element can also be formed with more than two phosphors. Allthat is important is that the conversion element is formed at least witha first phosphor and with a second phosphor.

The conversion element may have an absorption wavelength range.Electromagnetic radiation lying in the absorption wavelength range isabsorbed by the conversion element. The absorbed radiation can excitethe conversion element to re-emit secondary radiation. In this case, theabsorption wavelength range does not have to be the entire wavelengthrange in which the phosphor can absorb primary radiation and re-emitsecondary radiation, rather a section of this wavelength range can beinvolved.

The first phosphor of the conversion element may have, in the absorptionwavelength range, an absorption that decreases as the wavelengthincreases. That is to say that, within the absorption wavelength range,the first phosphor has a higher absorption and a lower absorption,wherein the first phosphor has the lower absorption at higherwavelengths than the higher absorption. By way of example, theabsorption of the first phosphor in the absorption wavelength rangefalls continuously as the wavelength increases.

The second phosphor may have, in the same absorption wavelength range,an absorption that increases as the wavelength increases. That is to saythat, within the absorption wavelength range, the second phosphor has ahigher absorption and a lower absorption, wherein the second phosphorhas the lower absorption at lower wavelengths than the higherabsorption. By way of example, the absorption of the second phosphor inthe absorption wavelength range rises continuously as the wavelengthincreases.

In other words, the absorption behavior of the two phosphors in theabsorption wavelength range is mutually opposite. As the wavelengthincreases, absorption of the first phosphor decreases, whereasabsorption of the second phosphor increases. The absorption wavelengthrange is then formed at least by a section of that wavelength range inwhich this statement is applicable.

The primary radiation may comprise wavelengths lying in the absorptionwavelength range. That is to say that the primary radiation compriseswavelengths lying in that wavelength range in which the absorptionbehavior of the first and second phosphors is mutually opposite.

The light-emitting diode may emit white mixed light composed of primaryradiation and secondary radiation. In this case, the mixed light has acolor temperature of at least 4000 K. By way of example, the colortemperature is then at most 7000 K. That is to say that the white mixedlight is cold-white light.

The light-emitting diode may comprise a light-emitting diode chip whichemits primary radiation in the spectral range of blue light during theoperation of the light-emitting diode. Furthermore, the light-emittingdiode comprises a conversion element which absorbs part of the primaryradiation and re-emits secondary radiation. In this case, the conversionelement comprises a first phosphor and a second phosphor. The firstphosphor has, in an absorption wavelength range, an absorption thatdecreases as the wavelength increases, and the second phosphor has, inthe same absorption wavelength range, an absorption that increases asthe wavelength increases. In this case, the primary radiation compriseswavelengths lying in the absorption wavelength range and thelight-emitting diode emits white mixed light composed of primaryradiation and secondary radiation and having a color temperature of atleast 4000 K.

Furthermore, we provide a conversion element for a light-emitting diode.The conversion element is suitable for use with a light-emitting diodechip. By way of example, the conversion element is suitable for alight-emitting diode. That means that all the features disclosed for theconversion element are also disclosed for the light-emitting diode andvice versa.

The conversion element is provided to absorb primary radiation and emitsecondary radiation. Preferably, the secondary radiation compriseshigher wavelengths than the primary radiation.

The conversion element may comprise a first phosphor and a secondphosphor, wherein the first phosphor has, in an absorption wavelengthrange, an absorption that decreases as the wavelength increases, and thesecond phosphor has, in the same absorption wavelength range, anabsorption that increases as the wavelength increases.

The wavelengths of the maximum emission intensity of the first andsecond phosphors may differ by at most 20 nm. In other words, the firstphosphor and the second phosphor have a different wavelength of themaximum emission intensity. In this case, the difference in thewavelength of the maximum emission intensity is, however, at most 20 nm.The difference is preferably at most 10 nm, particularly preferably atmost 7 nm.

In other words, the two phosphors emit light of the same color, whereinthe maximum in the emission of the two phosphors can be slightly shiftedrelative to one another.

The following examples relate both to the light-emitting diode and tothe conversion element.

The secondary radiation emitted by the conversion element may lie in thespectral range of yellow light. That is to say, in particular, that bothphosphors of the conversion element emitting electromagnetic radiationin the spectral range of yellow light, wherein the wavelengths of themaximum emission intensity can be shifted relative to one another, asdescribed above.

The wavelength of the maximum emission intensity of the second phosphormay be greater than that of the first phosphor. That is to say that thesecond phosphor has its maximum emission at a wavelength that is greaterthan the wavelength at which the second phosphor has its maximumemission.

The first phosphor may be based on europium (Eu) as a luminous centerand the second phosphor is based on cerium (Ce) as a luminous center.Preferably, the second phosphor based on cerium as a luminous center hasa wavelength of the maximum emission intensity that is somewhat greaterthan the wavelength of the maximum emission intensity of the firstphosphor based on Eu as a luminous center.

The maximum of the emission intensity of the primary radiation, that isto say of the electromagnetic radiation emitted by the light-emittingdiode chip, may lie between at least 440 nm and at most 470 nm,preferably between 445 nm and 460 nm. In this case, the wavelength rangeof the primary radiation preferably forms the absorption wavelengthrange in which the first phosphor has an absorption that decreases asthe wavelength increases, and the second phosphor has an absorption thatincreases as the wavelength increases.

The absorption of the conversion element may fall by at most 35% in theabsorption wavelength range, that is to say in particular in thewavelength range of at least 440 nm to at most 470 nm. In this case, theabsorption of the conversion element is the summed absorption of thephosphors of the conversion element.

The first phosphor and the second phosphor may be based on cerium as aluminous center, wherein the absorption wavelength range of one of thephosphors is shifted relative to the other phosphor by changing thecomposition of the host lattice of the phosphor. This results in totalin a wider absorption band than for the individual phosphors. Thegallium-containing system YAG:Ce and Y(Ga,Al)G:Ce is appropriate as anexample.

The weight ratio of the first phosphor in the conversion element to thesecond phosphor in the conversion element may be between at least 0.6and at most 1.5. By way of example, the following weight ratios of thefirst phosphor to the second phosphor are particularly preferred: 2:3,7:8, 1:1, 8:7, 3:2.

Such weight ratios of the first phosphor to the second phosphor make itpossible to provide a conversion element in which the absorption in theabsorption wavelength range of the conversion element is virtuallyconstant, that is to say hardly falls, for example. A light-emittingdiode comprising such a conversion element is therefore particularlyinsensitive to changes in the wavelength of the primary radiation.

The light-emitting diode may comprise at least two light-emitting diodechips, wherein the maximum of the emission intensity of two of thelight-emitting diode chips of the light-emitting diode differs from oneanother by at least 5 nm. That is to say that the two light-emittingdiode chips are not presorted particularly precisely, but rather have arelatively large difference in the dominant wavelength of their primaryradiation. A conversion element described here is disposed downstream ofthe light-emitting diode chips of the light-emitting diode. On accountof the wide, virtually uniform absorption of the conversion element,despite the use of light-emitting diode chips having a dominantwavelength greatly different from one another, a light-emitting diode isprovided which can emit white mixed light in a predeterminable,well-defined color locus range. The color locus of the white lightgenerated has hardly any spatial fluctuations despite the use ofdifferent light-emitting diode chips.

The light-emitting diode described here and also the conversion elementdescribed here are explained in greater detail below on the basis ofexamples and associated figures.

Elements that are identical, of identical type or act identically areprovided with the same reference symbols in the figures. The figures andthe size relationships of the elements illustrated in the figures amongone another should not be regarded as to scale. Rather, individualelements may be illustrated with an exaggerated size to enable betterillustration and/or to afford a better understanding.

Light-emitting diodes that emit white light can be produced from ablue-emitting light-emitting diode chip 1 and a yellow-emittingconversion element 34 as seen in FIGS. 10A to 10D. That is to say thatthe light-emitting diode chip 1 emits blue primary radiation, while theconversion element 34 emits yellow secondary radiation.

In this case, the conversion element 34 absorbs part of the blue light,this part then being re-emitted in the yellow spectral range. Thetransmitted part of the blue light with the converted yellow lighttogether produce the white color impression. The construction of thelight-emitting diode can be kept very compact if the blue light-emittingdiode chip 1 is enveloped with the conversion element 34 as shown inFIGS. 10B to 10D.

Blue light-emitting diode chips 1 are based, for example, on thematerial system GaInN. The emission wavelength can be set by the indium(In) content in a wide range of the visible spectrum, for example, fromapproximately 360 nm to approximately 600 nm. In this case, the spectralrange of 440 nm to 470 nm is preferably used for white light-emittingdiodes.

In the case of the LED phosphors, one material that is particularly wellsuited is cerium-doped YAG (Y₃Al₅O₁₂), or certain modifications with Gd,Tb or Ga. The cerium-doped phosphors have a strong absorption band inthe blue spectral range and emit in the yellow region, that is to sayare outstandingly suitable for white light-emitting diodes. However,other yellow-emitting phosphors based on europium as a luminous centeralso prove to be advantageous. These include, for example, theorthosilicates (Ca, Sr, Ba)SiO₄:Eu or the oxynitrides (Ca, Sr,Ba)Si₂O₂N₂:Eu.

The human eye reacts very sensitively to small color differences.Therefore, during production of white luminous means, attempts are madeto keep the color locus variation within a small bandwidth. In the caseof white light-emitting diodes, one important contribution to the colorlocus variation is the spectral variation of the light emitted by thelight-emitting diode chip 1. The variation of the emission wavelength inthe production process has a certain range. It may likewise belogistically advantageous to be able to mix light-emitting diodes havingdifferent emission wavelengths in the products.

FIG. 1 shows a series of spectra of blue light-emitting diode chips 1from the relevant spectral range. In this case, the emission spectra ofthe blue light-emitting diode chips extend over wavelengths of themaximum emission intensity, that is to say the dominant wavelengthsλ_(D) of at least 440 nm to at most 470 nm. In FIG. 1, the intensity Iis plotted against the wavelength λ.

The second spectral variation occurs in the application of thelight-emitting diode itself. Thus, the emission wavelength of alight-emitting diode chip shifts both with the operating current I andwith the operating temperature T.

In this respect, FIG. 2A shows the spectral variation during theoperation of a blue light-emitting diode chip 1 with the operatingcurrent I. The wavelengths of the maximum emission intensity shifttoward smaller wavelengths as the current I increases.

FIG. 2B shows the spectral variation during operation of a bluelight-emitting diode chip 1 with operating temperature T. Thewavelengths of the maximum emission intensity shift toward greaterwavelengths as the temperature T increases. Hence, the spectra becomewider.

The change in the spectrum of the blue light-emitting diode chip 1 alsohas effects on the color locus of the white light-emitting diode. Theabsorption behavior of the phosphors used is itself also spectrallydependent. As a result, the quantity of the absorbed blue and/orre-emitted yellow light changes, which leads to a blue and/or yellowshift in the white mixed light of the white LED.

In production, attempts are made to avoid the problem by carrying out apresorting of the semiconductors according to emission wavelength(so-called “binning”). However, such sorting is time- andcost-intensive, and it additionally leads to losses in yield as a resultof light-emitting diode chips that cannot be utilized. The requirementfor closely sorted groups is increasing. Hence, a supply bottleneck mayarise in the future.

Furthermore, in the field of light-emitting diode technology, waferlevel processes are also conceivable wherein wavelength sorting is notpossible, since, for example, a wafer comprising a multiplicity oflight-emitting diode chips is intended to be coated with a commonconversion element. In this case, therefore, tolerant processes mustprovide for the necessary accuracy.

In the field of light-emitting diode applications, too, color locusvariation poses problems. Thus, by way of example, a pulse widthmodulation is used for brightness dimming to avoid a color locus driftas a result of current density effects. Devices that are stable withrespect to color locus would make it possible to revert to simplercurrent-driven driving systems. The air conditioning of the devicescould also be dimensioned in a simpler manner.

The absorption and emission behavior of a second, cerium-doped phosphor4 is illustrated in greater detail in FIG. 3A. In curve a), theabsorption K is plotted against the wavelength λ. In curve b), theemission intensity E is plotted against the wavelength λ.

The absorption and emission behavior of a first, Eu-doped oxynitridephosphor 3 is illustrated in greater detail in FIG. 3B. In curve a), theabsorption K is plotted against the wavelength λ. In curve b), theemission intensity E is plotted against the wavelength λ.

To determine the spectra, the following should be noted:

-   -   The spectra of the blue light-emitting diode chips were measured        on (Ga, In)N-based light-emitting diodes. The emission spectra        of the phosphors were measured on powder samples. The        absorptance was able to be determined from reflection        measurements. The Kubelka-Munk method was used for evaluating        the data. The absorptance relates to the Kubelka-Munk parameter        K, which represents the attenuation in the propagation        direction.

A certain part of the change in the white color locus in the event of achange in the emission of the light-emitting diode chip 1 is based onthe color shift of the blue light per se. However, the greater part ofthe color locus shift is caused by the spectral dependence of theabsorption by the phosphor. As can be seen in FIGS. 3A and 3B, thephosphors have absorption edges that rise steeply precisely in the bluespectral range of relevance. Small spectral changes in the excitationtherefore have a great effect on the later color locus. The dependenciesare governed by the atomic structure of the phosphors and, unlike theemission wavelength, can scarcely be influenced. A small shift in theabsorption band is possible in the case of YAG-based phosphors, forexample, by adding gallium, but does not change anything about the basicform of the absorption curve.

FIG. 4 shows the color shift when using different emission wavelengthsfor the same conversion layer. In this case, FIG. 4 shows the calculatedcolor locus for light-emitting diode chips 1 having a different blueemission wavelength given the same configuration of the conversionelement. Curve a) was calculated for the first phosphor 3, and curve b)was calculated for the second phosphor 4.

The color space spanned is unacceptably large. Therefore, sorting andcontrol of the conversion element is necessary. However, even thatenables the required accuracies to be achieved only with difficulty.

For the cerium-doped garnet phosphor 4, the yellow component increasesas the emission wavelength increases, while for the Eu-doped oxynitride,the first phosphor 3, the yellow component decreases. This can also bediscerned from the compilation of the absorption bands for the firstphosphor 3, curve a), and the second phosphor 4, curve b), with theemission spectra for different blue light-emitting diode chips 1 (seeFIG. 5).

One concept of the conversion element and of a light-emitting diode is,then, that of using a phosphor mixture wherein the components have, inthe range of the blue light-emitting diode chip wavelength used, amutually opposite absorption behavior. Through a suitable choice of theconcentration ratios, it is thus possible to establish a wide constantabsorption band. Since the emission colors of the two phosphors areclose together, it is possible to use almost any desired concentrationswithout influencing the white point.

There is a differentiation with respect to warm-white light-emittingdiodes having color temperatures around 3000 K. In the case of thelatter, a phosphor mixture composed of a yellow and a red phosphor couldbe used. However, the concentration would not be freely selectable sincethe color locus would simultaneously have to be set by the ratio. Inthis case, by way of example, the proportion of the Eu-doped redphosphor would have to be chosen to be significantly smaller such thatthe variation in the absorption behavior cannot be obtained.

FIG. 6 shows the combination of cerium-doped second phosphor, curve b),and Eu-doped first phosphor, curve a). In the mixture, curve a+b), it ispossible to establish an almost constant absorption K for wavelengths of<460 nm. In the absorption wavelength range Δλ_(ab), in particular inthe wavelength range of at least 440 nm and at most 470 nm, that is tosay the absorption wavelength range Δλ_(ab), the absorption K of theconversion element 34 comprising first phosphor 3 and second phosphor 4falls by at most 35%.

The positive effect on the color locus variation can be seen in FIG. 7.Curves c1, c6 relate to the pure phosphors. In this case, only a smallportion of the possible excitation wavelengths is situated in the colorfield shown. That is different in the case of the phosphor mixturesused. Here the color loci for all emission wavelengths used are situatedwithin the diagram. It is even possible to maintain the colortemperature within a range of approximately 100 K (the depicted Juddstraight lines of the same color temperature have a distance of 100 K).The color loci lie within a window of Δcx=0.005, which represents a verynarrow distribution. Curves c2, c3, c4 and c5 show weight mixing ratiosof the second phosphor to the first phosphor of 7:8, 1:1, 8:7, and 3:2.Curve a) is the Planck curve. The wavelength separation between twomarkings is in each case 2.5 nm in FIG. 7.

The color locus shift with the operating current can also besignificantly reduced by using the phosphor mixture. In the case of aΔcx=0.001, the shift is virtually unmeasurable, and dimming of thelight-emitting diode is therefore possible without additional measures,without the color locus of the white mixed light being appreciablyshifted.

The concentrations to obtain a narrow distribution, in the case of thephosphors used, range around the ratio of 1:1 of the volume of the firstphosphor 3 to the volume of the second phosphor 4. A slight excess ofsecond phosphor 4, for example, YAG:Ce, obtains the least variation overthe entire range. If the blue wavelength range is restricted, that is tosay without the use of extremely long- and short-wave diodes, then aslight excess of first phosphor 3, for example, SiON:Eu, can also obtainnarrow distributions.

The indication of a concentration depends, of course, on theabsorptivity manifested by the phosphor. In the example shown, bothphosphors have the same maximum absorptivity, relative to the phosphorvolume, in the relevant wavelength range. Therefore, identicalconcentrations achieve the best result. However, it may also beexpedient to vary the doping concentration of one phosphor. Lower ceriumdopings result in an improved high-temperature behavior in YAG:Ce, forexample. The phosphor color is likewise set by the doping concentration.The concentration indications given here therefore relate to a lesserextent to the total mass of the phosphor, but rather to the content ofluminous centers.

FIG. 8 shows the color locus shift upon a change in the operatingcurrent I for conversion elements concerning the first phosphor 3 (curvea)), the second phosphor 4 (curve b)) and the first and second phosphors(curve a+b)).

The explanations considered here preferably relate to the color regiondesignated as “cold-white,” having color temperatures of between 4000 Kand 7000 K in the region of the Planckian color progression. In thiscase, the adherent color of the conversion element 34 is in the regionaround 570 nm, with a variation range of approximately +/−5 nm. Lowcolor temperatures require a longer emission wavelength, and colderwhite requires a lower wavelength. The emission color of thelight-emitting diode chips is intended to vary in the range of 440 nm to470 nm. A restricted range of approximately 445 nm to 460 nm ispreferred. Here, too, the light-emitting diodes will be chosen in thelonger-wave range for lower color temperatures.

For the selection of the phosphors, the cerium-doped garnet phosphorsare appropriate as second phosphors 4. A typical representative isYAG:Ce having an emission wavelength of 572 nm, for example. The coloris concomitantly determined by the cerium content; lightly dopedphosphors exhibit a short-wave shift. Other representatives are (Lu,Y)(Ga, Al)G:Ce having short-wave shifted emission and absorption, andalso (Gd, Y)AlG:Ce having long-wave shifted emission. It is possible tosubstitute yttrium with terbium or praesodymium instead of cerium.Combinations of the compositions are possible.

Various classes of the Eu²⁺-doped phosphors are appropriate as the firstphosphor 3 having a wavelength of the maximum emission intensity whichis lower than that of the second phosphor 4. Possible materials are thethiogallates (Mg, Ba, Sr)Ga₂S₄, although with preferably a greenishemission color. The orthosilicates (Ca, Mg, Ba, Sr)SiO₄ haverepresentatives having yellow emission. The class of oxynitrides (Ba,Sr, Ca)Si₂O₂N₂:Eu²⁺ is preferred. These phosphors emit in the yellowspectral range. One important selection criterion for this purpose isthe conversion efficiency at elevated temperature (temperaturequenching). At 150° C., a YAG:Ce_(0.02) still has 90% of its conversionefficiency at room temperature. The thiogallates and orthosilicates areat approximately 80%, and significantly lower at even highertemperatures. By contrast, at 150° C., the oxynitrides are still at 95%of their room temperature performance, and a system that can be usedeven at high temperatures can thus be assembled by combining garnet andoxynitride.

As an alternative to the traditional phosphors, it is also possible touse semiconductors or semiconductor nanoparticles since they exhibit anabsorption that increases toward shorter wavelengths. By way of example,the class of II/VI compound semiconductors (Zn, Mg, Cd)(S, Se), or (Ga,In)N, exhibits emission in the yellow region.

The emission color of the two different phosphors can lie in the yellowspectral range in one example. In a first example it would be attemptedto coordinate the emission wavelength of both phosphors with one anotheras well as possible. It is then unimportant which phosphor contributesto the emission to an increased extent. What is disadvantageous aboutthis method is that, as a result of the color locus shift of the bluelight-emitting diode chip, a certain color locus spreading in thered-green direction cannot be avoided. This method can therefore beadvantageously used at low color temperatures with a higher degree ofconversion, since the spreading decreases here.

In a second example it is appropriate for the emission wavelengths to beshifted relative to one another by a few nanometers, preferably by lessthan 7 nm. The second phosphor is preferably subjected to a long-waveshift. As a result, long-wave emitting chips are drawn downward in thecolor locus such that it is also possible to achieve a delimitation ofthe color locus in the red-green axis.

For more exact color locus control, it is also possible to use a mixtureof three or more phosphors, wherein the additional phosphors can againbelong to the class of cerium-doped or Eu-doped phosphors.

FIG. 9 shows the spectral profile of the white light-emitting diode forthe individual phosphors and the mixture (curve a+b)). The spectrum ofthe second phosphor (curve b)) has a full width at half maximum ofapproximately 100 nm. The spectrum of the first phosphor (curve a))exhibits a somewhat narrower band (approximately 70-80 nm). That has apositive effect on the visual efficiency since the maximum of the eyesensitivity is at 555 nm.

The color locus calculation for the light-emitting diode was also againeffected by the Kubelka-Munk method taking account of scattering,absorption and emission with full spectral dependence.

FIGS. 10A to 10D show examples of light-emitting diodes and conversionelements 34 in schematic sectional illustrations.

In a first example, FIG. 10A, the phosphor pairs are used in a mixture.For this purpose, the phosphor powders for forming the conversionelement 43 are weighed together in the correct ratio and subsequentlymixed into a matrix material 2, for example, a silicone or epoxy resinor a glass. This conversion element 43 is filled into the cavity of anLED, wherein the total concentration of the phosphor mixture iscoordinated with the height of the cavity, which is defined by thehousing basic body 5.

In a further form of application, FIG. 10B, the conversion element 34 isarranged around the light-emitting diode chip 1. For this purpose, byway of example, highly concentrated thin layers of the conversionelement 34 are produced. The phosphor can be injection-molded, printed,laminated or sedimented around the light-emitting diode chip 1. It isalso possible to produce the layer separately with subsequent adhesivebonding. The layer can be applied as a mixture, as illustrated in FIG.10C.

Besides the use of a mixture, it is also possible to use layer stacks asshown in FIG. 10D. In this case, by way of example, two films comprisingthe phosphors 3, 4 are combined. It is likewise possible to usecombinations of coating and volume potting. The order of the phosphorsis not of major importance since the phosphors do not mutually absorbone another.

It is furthermore also possible to use for the conversion element 34 acarrier composed of one of the phosphors, on which the other phosphor isarranged. By way of example, the carrier can consist of a cerium-dopedYAG ceramic, on which the second phosphor is deposited or introduced ina matrix material.

Our LEDs and conversion elements are not restricted to the examples bythe description. Rather, this disclosure encompasses any novel featureand also any combination of features, which in particular includes anycombination of features in the appended claims, even if the feature orcombination itself is not explicitly specified in the claims orexamples.

1. A light-emitting diode comprising: a light-emitting diode chip, which emits primary radiation in a spectral range of blue light during operation; a conversion element, comprising a first phosphor and a second phosphor which absorbs part of the primary radiation and re-emits secondary radiation, wherein the first phosphor has, in an absorption wavelength range(Δλ_(ab)), an absorption that decreases as the wavelength increases, and the second phosphor has, in the same absorption wavelength range (Δλ_(ab)), an absorption that increases as the wavelength increases; the primary radiation comprises wavelengths that lie in said absorption wavelength range (Δλ_(ab)), and the light-emitting diode emits white mixed light comprising primary radiation and secondary radiation and having a color temperature of at least 4000 K.
 2. The light-emitting diode according to claim 1, wherein the first phosphor and the second phosphor emit light of the same color, and the wavelengths of maximum emission intensity of the first and second phosphors are slightly shifted relative to one another.
 3. The light-emitting diode according to claim 1, wherein wavelengths of the maximum emission intensity of the first and second phosphors differ by at most 20 nm.
 4. The light-emitting diode according to claim 1, wherein the secondary radiation lies in the spectral range of yellow light.
 5. The light-emitting diode according to claim 1, wherein the wavelength of the maximum emission intensity of the second phosphor is greater than that of the first phosphor.
 6. The light-emitting diode according to claim 1, wherein the first phosphor is based on europium as a luminous center and the second phosphor is based on cerium as a luminous center.
 7. The light-emitting diode according to claim 1, wherein the second phosphor comprises (Gd,Lu,Y)(Al,Ga)G:Cer³⁺.
 8. The light-emitting diode according to claim 1, wherein the first phosphor comprises (Ca,Sr,Ba):SiO₄:Eu²⁺ and/or (Ca,Sr,Ba):Si₂O₂N₂:Eu²⁺.
 9. The light-emitting diode according to claim 1, wherein a maximum of the emission intensity of the primary radiation (λ_(D)) lies between at least 440 nm and at most 470 nm.
 10. The light-emitting diode according to claim 1, wherein the first phosphor and the second phosphor are based on cerium as a luminous center, and the absorption wavelength range (Δλ_(ab)) of one of the phosphors is shifted relative to the other phosphor by changing the composition of a host lattice of the phosphor.
 11. The light-emitting, diode according to claim 10, wherein one of the phosphors is or contains YAG:Ce and the other phosphor is or contains Y(Ga,Al):G:Ce.
 12. The light-emitting diode according to any of the preceding claim 1, wherein the absorption of the conversion element falls by at most 35% in the absorption wavelength range (Δλ_(ab)), in particular in the wavelength range of at lest 440 nm and to 470 nm.
 13. The light-emitting diode according to claim 1, wherein a weight ratio of the first phosphor to the second phosphor is 0.60 and to 1.5.
 14. The light-emitting diode according to claim 1, comprising two light-emitting diode chips, wherein a maximum of the emission intensity of electromagnetic radiation generated by the light-emitting diode chips during operation differs by at least 5 nm.
 15. A conversion element for a light-emitting diode, said conversion element provided to absorb a primary radiation and emit a secondary radiation, comprising: a first phosphor and a second phosphor, wherein the first phosphor has, in an absorption wavelength range (Δλ_(ab)), an absorption that decreases as the wavelength increases, and the second phosphor has, in the same absorption wavelength range (Δλ_(ab)), an absorption that increases as the wavelength increases, and wavelengths of the maximum emission intensity of the first and second phosphors differ by at most 20 nm.
 16. A light-emitting diode comprising: a light-emitting diode chip which emits primary radiation in a spectral range of blue light during operation; a conversion element comprising a first phosphor and a second phosphor which absorbs part of the primary radiation and re-emits secondary radiation, wherein the first phosphor has, in an absorption wavelength range (Δλ_(ab)), an absorption that decreases as the wavelength increases, and the second phosphor has, in the same absorption wavelength range (Δλ_(ab)), an absorption that increases as the wavelength increase; the primary radiation comprises wavelengths that lie in said absorption wavelength range (Δλ_(ab)); the light-emitting diode emits white mixed light comprising primary radiation and secondary radiation and having a color temperature of at least 4000 K; the first phosphor is based on europium as a luminous center and the second phosphor is based on cerium as a luminous center; and the weight ratio of the first phosphor to the second phosphor is 0.60 to 1.5. 